The joint between articles being coupled together, such as at the flanges terminating respective sections of fluid-carrying pipes, often must be sealed against leakage by the use of some type of gasket. The gasket material to be used must have appropriate resiliency and resistance against deterioration by the chemical and physical characteristics of the fluid, under the operating conditions of temperature and pressure to which the gasket will be exposed. When the pipes being coupled are to be used in sanitary or hygienic conditions additional criteria come into play such as ISO 2853 relating to stainless steel threaded couplings for the food industry that establish what steps must be taken to safeguard the sanitary conditions. The development of new products and processing methods often imposes increasingly strict requirements on operating conditions causing the coupling to be operated at the limit of its capability with an increasing risk of failure from the hygienic as well mechanical point of view. A 1995 report prepared by the Subgroup Pipe Couplings of the European Hygienic Equipment Design Group entitled "Hygienic Couplings" deals with the problems of joint sealing against contamination by micro-organisms. Among the points made in the report is that, to provide a bacteria-tight seal, valleys in the surface profile of the coupling joint must be filled by the elastomeric gasket material, typically by tightening the fastening device holding the coupling together, until sufficient contact pressure is applied. If too little contact pressure is applied by the fastening device, valleys in the surface profile will not be adequately filled to prevent accumulation of micro-organisms. Empirical data tends to show that with an elastomeric gasket having a Shore A hardness of 70.degree. a minimum contact pressure of 1.5 N/mm.sup.2 is required. This contact pressure corresponds to an elastomeric gasket being compressed by 15 per cent of its original thickness.
When a gasket is fabricated of elastomeric material, compressing one dimension of the gasket results in expansion of its other dimension, but the total volume of gasket material remains constant. Thus, assuming that a gasket having an original thickness of T is compressed an amount t (by tightening of the fastener), the gasket's width dimension W will increase by an amount w. It is easy to show that the percentage increase in the width dimension may be greater than the percentage decrease in the thickness dimension: The percentage decrease based on the original thickness may be expressed as: ##EQU1## while the percentage increase based on the original width may be expressed as ##EQU2## Applying the assumption that the gasket volume will remain constant means that: EQU (W+w)*(T-t)-WT (1)
From equation (1) it is seen that the fractional increase in width is given by: ##EQU3## For example, let it be assumed that ##EQU4## is 20%, equation (2) above shows that ##EQU5## will be 25%, i.e., the percentage increase in width is greater than the percentage decrease in thickness. This is important because sanitary standards dictate that this expansion be not so great as to cause undue projection into the pipe lumina as doing so may provide an obstruction or partial dam that could cause material to be accumulated from the fluid stream at the gasket's surface. Accordingly, the industry has recognized that the tightening force applied by the fastening device must be controlled and limited. The most common fastening device is the threaded bolt and nut combination and the conventional way of limiting the force applied to the gasket is to employ a torque wrench which indicates the foot-lbs of torque being applied when the fastener is made up. There are formulae for translating the torque applied to the fastening device to the amount of normal or axial compressive forces applied to the gasket. While the use of a torque wrench is quite common, it would be advantageous to provide a fastener which could be taken up with an ordinary wrench and which would nevertheless result in the desired amount of compressive stress being imparted to the gasket without fear of overstressing the gasket.